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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Space-based crystal growth and thermocapillary flow

The demand for larger crystals is increasing especially in applications associated with the electronic industry, where large and pure electronic crystals (notably silicon) are the essential material to make high-performance computer chips. Crystal growth under weightless conditions has been considered an ideal way to produce bigger and hopefully better crystals. One technique which may benefit from a microgravity environment is the float-zone crystal-growth process, a containerless method for producing high-quality electronic material. In this method, a rod of material to be refined is moved slowly through a heating device which melts a portion of it. Ideally, as the melt resolidifies it does so as a single crystal which is then used as substrate for building microelectronic devices. The possibility of contamination by contact with other material is reduced because of the 'float' configuration. However, since the weight of the material contained in the zone is supported by the surface-tension force, the size of the resulting crystal is limited in Earth-based productions; in fact, some materials have properties which prevent this process from being used to manufacture crystals of reasonable size. Consequently, there has been a great deal of interest in exploiting the microgravity environment of space to grow larger size crystals of electronic material using the float-zone method. In addition to allowing larger crystals to be grown, a microgravity environment would also significantly reduce the magnitude of convection induced by buoyancy forces during the melting state. This type of convection was once thought to be at least partially responsible for the presence of undesirable nonuniformities--called striations--in material properties observed in float-zone material. However, past experiments on crystal growth under weightless conditions found that even with the absence of gravity, the float-zone method sometimes still results striations. It is believed that another mechanism is playing a dominant role in the microgravity environment.

Shen, Yong-Hong↗
NASA NTRS1979

LSA Large Area Silicon Sheet Task Continuous Liquid Feed Czochralski Growth

A process for the continuous growth of crystals by the Czochralski method, suitable for producing single silicon crystals for use in solar cells was studied. Continuous growth is the growth of 100 Kg of single silicon crystals, 10 cm in diameter, from one container. A furnace with continuous liquid replenishment of the growth crucible, accomplished by a melt-down system and a liquid transfer mechanism, with associated automatic feedback controls was developed. Elements of the transfer system were further developed and tested during actual transfer runs. Considerable simplification of the heating element of the transfer tube was achieved. Accuracy and reliability of the temperature sensor, which is part of the power input control system for the transfer tube, was improved. Electrical and thermal effectiveness were increased while assembly of the transfer tube system was further simplified.

Fiegl, G.↗
DOE OSTI2020

Temperature-dependent phonon lifetimes and thermal conductivity of silicon by inelastic neutron scattering and ab initio calculations

In this paper, inelastic neutron scattering on a single crystal of silicon was performed at temperatures from 100 to 1500 K. These experimental data were reduced to obtain phonon spectral intensity at all wave vectors $\overset{→}{Q}$ and frequencies ω in the first Brillouin zone. Thermal broadenings of the phonon peaks were obtained by fitting and by calculating with an iterative ab initio method that uses thermal atom displacements on an ensemble of superlattices. Agreement between the calculated and experimental broadenings was good, with possible discrepancies at the highest temperatures. Distributions of phonon widths versus phonon energy had similar shapes for computation and experiment. These distributions grew with temperature but maintained similar shapes. Parameters from the ab initio calculations were used to obtain the thermal conductivity from the Boltzmann transport equation, which was in good agreement with experimental data. Despite the high group velocities of longitudinal acoustic phonons, their shorter lifetimes reduced their contribution to the thermal conductivity, which was dominated by transverse acoustic modes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1982

Heat-Exchanger Method of Crystal Growth

Large crystals of silicon are grown from melt, in either vacuum or pressurized atmosphere, without moving crucible, furnace, or anything else. Seed crystal is mounted on helium-cooled heat exchanger, which prevents seed from melting when furnace melts rest of silicon material in crucible; heat exchanger draws off heat from melt so that a solid ingot grows outward from seed in a regular crystal structure. Bottom of crucible is insulated so that heat exchanger cools only seed.

Khattak, C. P.↗
NASA NTRS1985

Impurities in silicon solar cells

Metallic impurities, both singly and in combinations, affect the performance of silicon solar cells. Czochralski silicon web crystals were grown with controlled additions of secondary impurities. The primary electrical dopants were boron and phosphorus. The silicon test ingots were grown under controlled and carefully monitored conditions from high-purity charge and dopant material to minimize unintentional contamination. Following growth, each crystal was characterized by chemical, microstructural, electrical, and solar cell tests to provide a detailed and internally consistent description of the relationships between silicon impurity concentration and solar cell performance. Deep-level spectroscopy measurements were used to measure impurity concentrations at levels below the detectability of other techniques and to study thermally-induced changes in impurity activity. For the majority of contaminants, impurity-induced performance loss is due to a reduction of the base diffusion length. From these observations, a semi-empirical model which predicts cell performance as a function of metal impurity concentration was formulated. The model was then used successfully to predict the behavior of solar cells bearing as many as 11 different impurities.

Hopkins, R. H.↗
DOE OSTI2019

Tailoring the crystal growth of quartz on silicon for patterning epitaxial piezoelectric films

Epitaxial films of piezoelectric α-quartz could enable the fabrication of sensors with unprecedented sensitivity for prospective applications in electronics, biology and medicine. However, the prerequisites are harnessing the crystallization of epitaxial α-quartz and tailoring suitable film microstructures for nanostructuration. Here, we bring new insights into the crystallization of epitaxial α-quartz films on silicon (100) from the devitrification of porous silica and the control of the film microstructures: we show that by increasing the quantity of devitrifying agent (Sr) it is possible to switch from an α-quartz microstructure consisting of a porous flat film to one dominated by larger, fully dense α-quartz crystals. We also found that the film thickness, relative humidity and the nature of the surfactant play an important role in the control of the microstructure and homogeneity of the films. Via a multi-layer deposition method, we have extended the maximum thickness of the α-quartz films from a few hundreds of nm to the μm range. Moreover, we found a convenient method to combine this multilayer approach with soft lithography to pattern silica films while preserving epitaxial crystallization. This improved control over crystallization and the possibility of preparing patterned films of epitaxial α-quartz on Si substrates pave the path to future developments in applications based on electromechanics, optics and optomechanics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2014

Investigation of Backside Textures for Genesis Solar Wind Silicon Collectors

Genesis solar wind collectors were comprised of a suite of 15 types of ultrapure materials. The single crystal, pure silicon collectors were fabricated by two methods: float zone (FZ) and Czochralski (CZ). Because of slight differences in bulk purity and surface cleanliness among the fabrication processes and the specific vendor, it is desirable to know which variety of silicon and identity of vendor, so that appropriate reference materials can be used. The Czochralski method results in a bulk composition with slightly higher oxygen, for example. The CZ silicon array wafers that were Genesis-flown were purchased from MEMC Electronics. Most of the Genesis-flown FZ silicon was purchased from Unisil and cleaned by MEMC, although a few FZ wafers were acquired from International Wafer Service (IWS).

Gonzalez, C. P.↗
NASA NTRS1979

LSA Large Area Silicon Sheet Task. Continuous Liquid Feed Czochralski Growth

The design and development of equipment and processes to demonstrate continuous growth of crystals by the Czochralski method suitable for producing single silicon crystals for use in solar cells is presented. The growth of at least 150 kg of mono silicon crystal, 150 mm in diameter is continuous from one growth container. A furnace with continuous liquid replenishment of the growth crucible, accomplished by a meltdown system with a continuous solid silicon feed mechanism and a liquid transfer system, with associated automatic feedback controls is discussed. Due to the silicon monoxide build up in the furnace and its retarding effect on crystal growth the furnace conversion for operation in the low pressure range is described. Development of systems for continuous solid recharging of the meltdown chamber for various forms of poly silicon is described.

Fiegl, G.↗
NASA NTRS2009

Survey Analysis of Materials Processing Experiments Aboard STS-47: Spacelab J

This Technical Memorandum (TM) is a survey outline of materials processing experiments aboard Space Shuttle Mission STS-47: Spacelab J, a joint venture between NASA and the National Space Development Agency of Japan. The mission explored materials processing experiments including electronics and crystal growth materials, metals and alloys, glasses and ceramics, and fluids. Experiments covered include Growth of Silicone Spherical Crystals and Surface Oxidation, Growth Experiment of Narrow Band-Gap Semiconductor Lead-Tin-Tellurium Crystals in Space, Study on Solidification of Immiscible Alloys, Fabrication of Very-Low-Density, High-Stiffness Carbon Fiber/Aluminum Hybridized Composites, High Temperature Behavior of Glass, and Study of Bubble Behavior. The TM underscores the historical significance of these experiments in the context of materials processing in space.

Sharpe, R. J.↗
NASA NTRS1982

Single crystal growth of upgraded metallurgical silicon by HEM for photovoltaic applications

Commercially available metallurgical grade (MG) silicon has high B and P content which is not reduced significantly by directional solidification. By choosing high purity raw materials for an experimental Submerged Electrode Arc Furnace, most of the impurities are reduced to 10 ppmw. Directional solidification of upgraded metallurgical grade (UMG) silicon by the Heat Exchanger Method (HEM) has produced 16 cm x 16 cm cross section ingots with nearly single crystal structure. The main problem encountered during directional solidification was SiC impurities dispersed through the structure. Solar cells fabricated from UMG silicon that was directionally solidified twice by HEM have shown up to 12.33% (am1) conversion efficiency.

Khattak, C. P.↗
DOE OSTI2024

Controlled Spalling of 4H Silicon Carbide with Investigated Spin Coherence for Quantum Engineering Integration

We detail scientific and engineering advances which enable the controlled spalling and layer transfer of single crystal 4H silicon carbide (4H-SiC) from bulk substrates. 4HSiC’s properties, including high thermal conductivity and a wide bandgap, make it an ideal semiconductor for power electronics. Moreover, 4H-SiC is an excellent host of solid-state atomic defect qubits for quantum computing and quantum networking. Because 4H-SiC substrates are expensive (due to long growth times and limited yield), techniques for removal and transfer of bulk-quality films are desirable for substrate reuse and integration of the separated films. In this work, we utilize updated approaches for stressor layer thickness control and spalling crack initiation to demonstrate controlled spalling of 4H-SiC, the highest fracture toughness crystal spalled to date. We achieve coherent spin control of neutral divacancy (VV 0 ) qubit ensembles and measure a quasi-bulk spin T 2 of 79.7 μs in the spalled films.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1979

Process for growing thin polished silicon sheets

Single-crystal sheets pulled from polten silicon floating on refractory melt require no slicing or polishing. Possible materials are chlorides and fluorides of barium, calcium, and strontium. Refractory melt may be able to absorb impurities from silicon and thus obviate postgrowth purification.

Radics, C. C.↗
NASA NTRS1972

Economic analysis of crystal growth in space

Many advanced electronic technologies and devices for the 1980's are based on sophisticated compound single crystals, i.e. ceramic oxides and compound semiconductors. Space processing of these electronic crystals with maximum perfection, purity, and size is suggested. No ecomonic or technical justification was found for the growth of silicon single crystals for solid state electronic devices in space.

Ulrich, D. R.↗
NASA NTRS1989

Semiempirical Model Would Control Czochralski Process

Semiempirical mathematical model proposed for control of growth of single crystals of silicon by Czochralski process. Expresses dependence of pulling rate and shape of liquid/solid interface upon important process variables; radius of growing crystal, temperature of crucible, level of melt, and height of exposed portion of crucible wall. Necessary to control shape of interface in manner consistent with other variables, to maintain radially uniform concentration of dopant, and reduce thermally induced stresses in vicinity of interface. Used to simulate complete growth cycles without requiring excessive computer time consumed by rigorous finite-element modeling.

Dudukovic, M. P.↗
NASA NTRS1984

Mechanisms limiting the performance of large grain polycrystalline silicon solar cells

The open-circuit voltage and short-circuit current of large-grain (1 to 10 mm grain diameter) polycrystalline silicon solar cells is determined by the minority-carrier diffusion length within the bulk of the grains. This was demonstrated by irradiating polycrystalline and single-crystal (Czochralski) silicon solar cells with 1 MeV electrons to reduce their bulk lifetime. The variation of short-circuit current with minority-carrier diffusion length for the polycrystalline solar cells is identical to that of the single-crystal solar cells. The open-circuit voltage versus short-circuit current characteristic of the polycrystalline solar cells for reduced diffusion lengths is also identical to that of the single-crystal solar cells. The open-circuit voltage of the polycrystalline solar cells is a strong function of quasi-neutral (bulk) recombination, and is reduced only slightly, if at all, by grain-boundary recombination.

Culik, J. S.↗
NASA NTRS1982

Crucible Grows Wide Silicon Ribbon

Inexpensive manufacture of solar cells may require quality silicon ribbon crystals. One way to produce them is by growing wide dendritic webs, which can be very long and have high structural perfection. Dendrites grow from supercooled melt, so width of ribbon depends on how wide a region of supercooled molten silicon can be maintained in crucible. Elongated geometry of suscepter/crucible/lid assembly allows molten silicon to supercool over a wider region -- a necessary condition for the growth of wide dendritic crystal ribbon.

Seidensticker, R.↗
NASA NTRS1987

Advancements in silicon web technology

Low defect density silicon web crystals up to 7 cm wide are produced from systems whose thermal environments are designed for low stress conditions using computer techniques. During growth, the average silicon melt temperature, the lateral melt temperature distribution, and the melt level are each controlled by digital closed loop systems to maintain thermal steady state and to minimize the labor content of the process. Web solar cell efficiencies of 17.2 pct AM1 have been obtained in the laboratory while 15 pct efficiencies are common in pilot production.

Hopkins, R. H.↗